CARSON CITY, Nev. — The shooter who killed four and injured seven in a rampage at a Nevada IHOP restaurant last month was using a weapon that was altered to function as an illegal machine gun, police said Wednesday.

Eduardo Sencion, 32, entered the IHOP in Carson City on Sept. 6 just before 9:00am local time and began shooting people with what appeared to be an assault rifle before turning the gun on himself.

An investigation by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) determined that Sencion had fired about 60 rounds using a Norinco Mak 90, a Chinese-made variation of the Russian-made AK-47.

That type of semi-automatic gun is not illegal in Nevada in its manufactured state, but Carson City Sheriff Ken Furlong said in a statement Wednesday that the weapon was altered to function as an automatic weapon.

“It has been unlawful since 1934 (National Firearms Act) for civilians to own machine guns without special permission from the U.S. Treasury Department,” Furlong’s statement said. “Machine guns are subject to a tax every time their ownership changes from one federally registered owner to another, and each weapon is subject to tax when it is made and it must be registered with ATF.”

Records showed that the weapon was last sold to an unknown buyer by a private party in California over five years ago, according to Furlong’s statement, which added that ATF officials described the gun’s alteration as “professionally gunsmithed.”

Authorities are continuing to investigate the source of the gun’s alterations and trace its history of ownership.

Among those inside the IHOP at the time of the massacre were five National Guard members — two women and three men — who were having an informal meeting about four miles (6.5km) from the Guard’s complex. All five of the National Guardsmen were shot and three died from their wounds.

It remained unclear if they were specifically targeted by Sencion, who had a history of mental illness but no prior criminal record.